Other
Dr. Brian Blood: Music Theory and History Online
An exhaustive resource with everything concerning music theory. From ear training to music history, this site covers it all.
Naxos Digital Services
Naxos Digital Services: Education Glossary of Music Terms
Classical music label Naxos provides brief definitions of musical terms.
Other
Tonalsoft: Encyclopedia of Microtonal Music Theory
Find an online encyclopedia specializing in microtonal music theory.
Ducksters
Ducksters: Music for Kids: Musical Terms and Glossary
Terms and words used in music. Learn about musical terminology.